Some Basic Aspects of Fluid Dynamics

Fluid dynamics, as the name implies, deals with the dynamic behavior of fluids, i.e., gases and liquids. We shall confine ourselves to the behavior of liquids in general and electrolytes in particular. When a liquid flows through a channel or along a surface, you can distinguish between two extremes of types of flow laminar flow and turbulent flow. In the former, the flow is streamline, i.e., there is no bulk motion perpendicular to the direction of flow and any transfer is due to the motion of single molecules. In the latter, fluid in the form of eddies moves rapidly in random directions across the direction of flow. 

FIGURE 2.1. Velocity profile for fully developed turbulent flow, u = Fluid velocity d = boundary layer thickness 3p = thickness of Prandtl boundary layer. 

FIGURE 2.2. Formation of boundary layers, (a) Initial laminar layer (S = boundary layer thickness) (b) formation of buffer layer (c) formation of turbulent layer. 

The behavior of the boundary layer explains the character of flow in the channel formed by the two electrodes. When the liquid enters the channel it immediately begins to form a boundary layer at the inside of the two electrodes and as it thickens it closes in on itself to meet in a cusp (Fig. 2.3). Before the cusp the flow pattern changes progressively as the boundary layer builds up this region is called the entrance region. Beyond the cusp, the flow pattern suffers no further change and is called fully developed. 

FIGURE 2.4. Types of fully developed flow, (a) Laminar (b) transitional (c) turbulent. 

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